ISO 11783-2 — the physical layer

Part 2 is the copper. It fixes the bus that every higher layer assumes exists: a single twisted pair running at a defined speed, terminated correctly, with bit timing chosen so dozens of ECUs spread across a long machine all agree on where each bit starts and stops.

Why this exists

A field machine is an electrically brutal place — long harnesses, big motors, welding repairs in a shed. The physical layer is the agreement that makes a robust, deterministic bus out of that environment, and that lets a stranger’s ECU plug in and just work at the same bitrate and timing as everyone else.

The profile

   CAN_H ──┬───────┬───────┬───────┬──
   CAN_L ──┴───────┴───────┴───────┴──
           │       │       │       │
         ECU 1   ECU 2   ECU 3    GNSS

   • 250 kbit/s   • terminated at both ends
   • twisted pair • defined bit timing + sample point

machbus does not toggle pins — a transport/driver owns the hardware — but it validates the profile: a wrong bitrate, an out-of-range sample point, or inconsistent explicit bit-timing segments are rejected before they can be mistaken for a compliant bus.

The one idea that matters upward: arbitration

CAN’s defining trick is non-destructive, priority-based arbitration. When two nodes transmit simultaneously, the bus ANDs their bits: a dominant 0 overwrites a recessive 1. The node sending the lower identifier wins and continues; the loser detects the mismatch and backs off — no frame is destroyed.

   ECU X  transmits  1 0 1 1 0 …
   ECU Y  transmits  1 0 1 0 …        ← differs at bit 4
   bus    carries    1 0 1 0 …        ← Y's dominant 0 wins
                            ▲
                     X sees bus ≠ what it sent → X yields, retries later

This “lowest number wins, losers retry cleanly” behavior is the hardware foundation for two things higher up: message priority (low priority value = wins the bus) and NAME-based address claiming (low NAME = wins the address). The same rule, all the way up the stack.

From concept to code

See also

• SAE J1939 heritage — what those arbitrated bits mean.
• ISO 11783-3 — data link & transport — moving more than 8 bytes over this wire.